Underfill integration for optical packages

ABSTRACT

The application discloses an apparatus comprising an optical die flip-chip bonded to a substrate and defining a volume between the optical die and the substrate, the optical die including an optically active area on a surface of the die facing the substrate, an optically transparent material occupying at least those portions of the volume substantially corresponding with the optically active area, and an underfill material occupying portions of the volume not occupied by the optically transparent material. Also disclosed is a process comprising flip-chip bonding an optical die to a substrate, the optical die including at least one optically active area on a surface thereof facing the substrate, dispensing an optically transparent material between the optical die and the substrate, wherein the optically transparent material covers the at least one optically active area, dispensing an underfill material in the volume between the optical die and the substrate not occupied by the optically transparent material, and curing the optically transparent material and the underfill material. Other embodiments are described and claimed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/606,092, filed on Jun. 24, 2003 now U.S. Pat. No. 7,042,106, andclaims priority therefrom under 35 U.S.C. § 120. The priorityapplication is currently pending.

TECHNICAL FIELD

The present invention relates generally to flip-chip bonding and inparticular, but not exclusively, to integration of flip-chip bondedoptical packages.

BACKGROUND

Flip-chip bonding is a well-known technology in which semiconductor diesor packages are turned upside down and attached to a substrate (e.g., asemiconductor wafer or printed circuit board (PCB)) using solder ballsor joints instead of being attached in more conventional ways, such asbeing placed right side up and attached to a substrate using perimeterbonding wires. FIG. 1 illustrates a flip-chip bonded configuration 100,in which a die 102 is mounted to a substrate 106 via flip-chip bonding.The die 102 is suspended above the substrate 106, is affixed to thesubstrate by a plurality of solder balls 110, and has a lower surface104 facing the substrate, creating a volume 105 between the surface 104of the die and the substrate surface 108.

One common problem with flip-chip bonding occurs due to differentialthermal expansion between the substrate 106 and the die 102. In manydevices, the substrate 106 has a coefficient of thermal expansion thatis substantially different than the coefficient of thermal expansion ofthe die 102. As with most semiconductor devices, when the device 100operates it experiences a substantial increase in temperature; thetemperature rise is directly related to the amount of electrical energyused by the device, which is eventually turned into heat. Thedifferences in coefficient of thermal expansion between the die and thesubstrate, coupled with large temperature increases, result insubstantially different amounts of thermal expansion and contraction inthe substrate and the die. Since the die is rigidly connected to thesubstrate by the plurality of solder balls 110, the loads created by thedifferential thermal expansions are carried entirely by the solderballs. Because the solder balls 110 are typically very small, the resultis a high stress concentration in the solder balls. These stressconcentrations can result in premature failure of the solder balls and,consequently, premature failure of the entire device 100.

FIG. 2 illustrates one approach that has been used to reduce theproblems caused by differential thermal expansion of the die and thesubstrate. In this approach, a material known as an “underfill” 202 isdispensed into the volume defined by the lower surface 104 of the dieand the upper surface 108 of the substrate 106. The underfill material202 is fairly rigid when cured, such that by filling the volume 105 withunderfill material any loads arising from the differential thermalexpansion of the substrate and the die, as well as any other mechanicalforces that may be applied to the die, are transferred into both thesolder balls and the underfill material. Since the applied loads are nowcarried over a substantially larger area (i.e., the area of the solderballs plus the area of the underfill material), the resulting stressesare lower and stress concentrations at the solder balls are eliminated.

Although the approach of using underfill is beneficial, it suffers fromsome disadvantages. Among those disadvantages is that the underfillapproach is incompatible with dies including optical devices. Underfillmaterials are typically opaque, and therefore cannot be used in deviceshaving optically active areas on the lower side 104 of the substratebecause the underfill material would absorb any radiation radiating fromor being received by an optically active area on the lower side 104 ofthe die, thus rendering useless the optically active area. In addition,most optical devices that would be used for the optically active areaare very delicate. Since underfill materials are designed to be rigid(e.g., having a modulus of elasticity, or Young's modulus, of 7-10 GPa)so they can take up thermal and other loads, the underfill material willtransfer loads to the optically active area and can damage the area.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a side elevation drawing of a die flip-chip bonded onto asubstrate, such as a printed circuit board.

FIG. 2 is a side elevation drawing of die flip-chip bonded onto asubstrate and including an underfill material sandwiched between the dieand the substrate.

FIG. 3A is a plan view drawing of an optical device including an opticaldie flip-chip bonded onto a substrate.

FIG. 3B is a side elevation drawing of an optical device including anoptical die flip-chip bonded onto a substrate.

FIGS. 4A-4D are side elevation drawings of an embodiment of an inventiveprocess for integrating a flip-chip bonded die to a substrate usingunderfill techniques.

FIG. 5 is a plan view of an embodiment of an optical system including apair of dies flip-chip bonded to a substrate and integrated thereto asshown in the embodiment depicted in FIG. 4A-4D.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Embodiments of a apparatus and method for underfill integration ofoptical packages are described herein. In the following description,numerous specific details are described to provide a thoroughunderstanding of embodiments of the invention. One skilled in therelevant art will recognize, however, that the invention can bepracticed without one or more of the specific details, or with othermethods, components, materials, etc. In some instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring aspects of the invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, appearancesof the phrases “in one embodiment” or “in an embodiment” in thisspecification do not necessarily all refer to the same embodiment.Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments.

FIGS. 3A and 3B illustrate an embodiment of an optical device 300. Asseen in plan view in FIG. 3A, the optical device 300 comprises anoptical die 302 flip chip bonded to a substrate 304 using solder balljoints 303. An optical component such as a waveguide 306 is insertedbetween the optical die 302 and the substrate 304, and occupies at leastpart of the volume between the optical die 302 and the substrate 304. Inaddition to having a portion extending underneath the die 302, a portionof the waveguide projects from underneath the die 302 and extends, forexample, to another component (see FIG. 5) to form an opticalinterconnect between components.

The optical die 302 is affixed to the substrate 304 using solder balljoints 303, which both attach the optical die 302 to the substrate 304and suspend the optical die above the substrate, such that there is avolume defined in part by the substrate 302 and the surface 308 of thedie facing the substrate. The optical die 302 includes at least oneoptically active area 310 on the surface 308. Examples of opticallyactive components that can be found on the die 302 include radiationsources such as a vertical cavity surface emitting laser (VCSEL), orphoto-detectors such as PINs, photodiodes and the like. Otherembodiments can, of course, include more, less or different opticallyactive areas than those listed. Similarly, other embodiments couldinclude additional optically active areas positioned on other surfacesof the optical die 302, for example on the surface of the die not facingthe substrate.

The waveguide 306 is positioned so that at least a portion of thewaveguide occupies at least part of the volume between the substrate 304and the lower surface 308 of the optical die. The remaining portion ofthe waveguide projects out from under an edge 314 of the optical die 302and can extend, for example, to another optical die (see FIG. 5) to forman optical interconnect between optical dies. The width of the waveguideis such that it fits between a pair of the solder ball joints 303.Although only one waveguide is illustrated, in other embodimentsmultiple waveguides could be used. For example, multiple waveguides canbe used to carry signals in different directions to or from a singleoptically active area 310, or multiple waveguides can be used to carrymultiple signals to and from multiple optically active areas.

FIG. 3B illustrates a side elevation of the optical device 300 shown inplan view in FIG. 3A. The waveguide 306 comprises an opticallytransparent portion 305 surrounded by cladding 307. A portion of thewaveguide 306 is positioned in the volume between surface 308 of theoptical die 302 and the substrate, such that its highly polished angledend 309 is positioned to either receive radiation from the opticallyactive area 310 and reflect it into the optically transparent portion305 of the waveguide (e.g., where the optically active area is aradiation source), or to receive radiation from the opticallytransparent portion of the waveguide and reflect it into the opticallyactive area 310 (e.g., where the optically active area 310 is aradiation detector). The remaining portion of the waveguide projects outfrom under an edge 314 of the optical die 302 and can extend, forexample, to another optical die (not shown) to form an opticalinterconnect between optical dies.

FIGS. 4A through 4D together illustrate an embodiment of a process forintegrating optical dies on a substrate such as a printed circuit board(PCB) using underfill. FIG. 4A illustrates the optical device 300, whichcomprises an optical die 302 flip-chip bonded onto a substrate 304. Inone embodiment of the device 300, the optical die 302 can be made ofGallium Arsenide (GaAs), which has a coefficient of thermal expansion ofabout 5.8 ppm/° C., while the substrate 304 can be a printed circuitboard (PCB) typically made of materials having a higher coefficient ofthermal expansion (alpha) of about 17 ppm/° C. Because of this mismatchin thermal expansion coefficients, and because during operation thedevice 300 can typically endure temperature rises of 100° C. or more, ifthe device 300 operates as it is shown in FIG. 4A it will be subject tolarge stress concentrations because there is no underfill between theoptical die 302 and the substrate 304 to carry some of the thermallyinduced stresses. Additionally, without underfill debris and moisturecan enter the volume between the die and the substrate and damage orimpair the optically active area of the die.

FIG. 4B illustrates a first step in the embodiment of the process. Abead of optically transparent material 402 is dispensed along the edge314 of the optical die. As its name suggests, the optically transparentmaterial should be optically transparent (i.e., it should absorb aminimum amount of radiation) at the operational wavelength of theoptically active area 310. In one embodiment in which the opticallyactive area 310 is a VCSEL the operational wavelength is typically 850nm, although VCSELs can operate at other wavelengths, and in otherembodiments the optically active area can be a different type of deviceoperating at the same or different wavelengths. In addition to beingoptically transparent, the optically transparent material 402 shouldhave a low coefficient of thermal expansion, and should have a modulusof elasticity (i.e., Young's modulus E) that is substantially lower thanconventional underfill materials. The material 402 should also have arefractive index substantially equal to the refractive index of thewaveguide 306 to prevent or minimize insertion losses. In oneembodiment, the optically transparent material can be a silicon-basedmaterial such as a silicon adhesive, although in other embodimentsmaterials such as acrylic or acrylic-based materials or certain epoxiescan be used.

FIG. 4C illustrates the next step in the embodiment of the process.Because of the small scales involved and the small distance between thewaveguide 306 and the surface 308 of the die, after the bead of theoptically transparent material 402 is applied along the edge 314 of thedie 302, capillary action causes the optically transparent material tobe drawn into the volume between the waveguide 306 and the surface 308of the die. In the embodiment shown, the capillary action draws theoptically transparent material far enough under the die such that itfills the entire volume corresponding to the overlap area between thedie and the waveguide. At the very least, the optically transparentmaterial should occupy the volume between the optically active area 310and the waveguide, so that radiation leaving or entering the waveguideor the optically active area 310 travels though the opticallytransparent material. In addition to the above-mentioned requirementsfor the optically transparent material, because the embodiment of themethod relies on capillary action to draw the material 402 into thevolume between the die and the waveguide, this creates an upper limit onthe viscosity of the material 402. There is, however, no lower limit onthe permissible viscosity of the material. When the opticallytransparent material occupies the desired areas between the die and thewaveguide, the optically transparent material can be cured beforecontinuing with the process, or can be cured after the conventionalunderfill is dispensed (see below).

FIG. 4D illustrates the next step in the embodiment of the process. Aconventional underfill material 404 is dispensed into the portion of thevolume between the die and the substrate and the waveguide not alreadyoccupied by the optically transparent material. In the embodiment shown,the conventional underfill material occupies the portion of the volumebetween the substrate 304 and the surface 308 of the die, while theoptically transparent material 402 occupies the portion of the volumebetween the waveguide and the surface 308 of the die. The conventionalunderfill 404 and the optically transparent material come into contactalong an interface 406. The position of the interface 406 must be suchthat none of the conventional underfill material comes into contact withthe optically active area 310 or impeded the transmission of radiationbetween the optically active area 310 and the waveguide. Although anyunderfill material can be used, typical conventional underfill materialsare a combination of a resin and a filler mixed in a proportion thatgives the desired or required physical properties. In one embodiment,the resin is an epoxy with a high coefficient of thermal expansion(e.g., 50-70 ppm/° C.) while the filler is silicon dioxide (SiO₂), whichhas a substantially lower coefficient of thermal expansion (e.g., 0.5ppm/° C.); the resulting underfill material has a coefficient of thermalexpansion of about 25 ppm/° C. and a modulus of elasticity of about 7-10GPa. Since conventional underfill material is opaque and should not playa role in the function of the optically active area, there are nooptical requirements for the conventional underfill as there are for theoptically transparent material.

Once the optically transparent material 402 and the underfill material404 have been dispensed into the volume between the optical die 302 andthe substrate and waveguide, both the material and the underfill must becured. In one embodiment, after they have been dispensed both theadhesive and the underfill are cured by placing the entire device 300 inan oven or autoclave and subjecting it to temperatures not exceeding180° C. for an amount of time not exceeding one hour. In otherembodiments, however, higher or lower temperatures and longer or shortercuring times can be used, as long as the curing temperature and time arenot sufficient to harm the optically active area or any other component.In addition, in other embodiments the optically transparent material 402and the underfill 404 need not be cured simultaneously. Instead, theoptically transparent material can be cured first followed by dispensingand curing of the underfill or, alternatively, the underfill can bedispensed and cured first followed by dispensing and curing of theoptically transparent material. Although generally more expensive andtime consuming, separate curing of the optically transparent materialand the underfill can have advantages, such as preventing any mixing ofoptically transparent material and underfill at the interface 406 beforecuring.

FIG. 5 illustrates an embodiment of a system 500 including a first die502 and a second die 504 flip-chip bonded to a substrate 501. The die502 is coupled to a signal source 506 and includes an optically activearea 508 that is an optical source. Similarly, the second die 504 iscoupled to a signal destination 510 and includes an optically activearea 512 that in this embodiment is an optical detector. A waveguide 514extends between the first die 502 and the second die 504, and ispositioned between each die and the substrate such that it can receivean optical signal from or transmit an optical signal to the opticallyactive area of each die. Both dies 502 and 504 are flip-chip bonded tothe substrate 501 and are integrated thereto using an opticallytransparent material and an underfill, substantially as shown in FIG.4D. In operation, the signal source 506 generates a signal that istransmitted to the optical die 502. The optically active area 508 turnsthe signal into an optical signal and launches it into the waveguide514. The signal travels through the waveguide to the optically activearea 512, which detects the signal and transmits it to the signaldestination 510.

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. These modifications can bemade to the invention in light of the above detailed description.

The terms used in the following claims should not be construed to limitthe invention to the specific embodiments disclosed in the specificationand the claims. Rather, the scope of the invention is to be determinedentirely by the following claims, which are to be construed inaccordance with established doctrines of claim interpretation.

1. A process comprising: flip-chip bonding an optical die to asubstrate, the optical die including at least one optically active areaon a surface thereof facing the substrate; dispensing an opticallytransparent material between the optical die and the substrate, whereinthe optically transparent material covers the at least one opticallyactive area; dispensing an underfill material in the volume between theoptical die and the substrate not occupied by the optically transparentmaterial; and curing the optically transparent material and theunderfill material.
 2. The process of claim 1 wherein curing theoptically transparent material and the underfill material comprisessimultaneously curing the optically transparent material and theunderfill material.
 3. The process of claim 1 wherein curing theoptically transparent material and the underfill material comprisesfirst curing the optically transparent material and then curing theunderfill material.
 4. The process of claim 1 wherein curing theoptically transparent material and the underfill material comprisesfirst curing the underfill material and then curing the opticallytransparent material.
 5. The process of claim 1 wherein the opticallytransparent material can be fully cured at temperatures less than orequal to 180° C.
 6. The process of claim 1 wherein the optically activearea is a detector or a source.
 7. The process of claim 1 wherein theoptically transparent material has a low modulus of elasticity.
 8. Theprocess of claim 1 wherein the optically transparent material has arefractive index of approximately 1.5.
 9. The process of claim 1 whereinthe optically transparent material is an adhesive.
 10. The process ofclaim 1 wherein the optically transparent material is silicone-based.11. A process comprising: flip-chip bonding an optical die to asubstrate, the optical die including at least one optically active areaon a surface thereof facing the substrate; inserting at least part of anoptical component between the optical die and the substrate to carry anoptical signal to or receive an optical signal from the optically activearea; dispensing an optically transparent material between the opticaldie and the optical component, wherein the optically transparentmaterial occupies at least the volume between the optically active areaand the optical component; dispensing an underfill material in thevolume between the optical die and the substrate not occupied by theoptically transparent material; and curing the optically transparentmaterial and the underfill material.
 12. The process of claim 11 whereinthe optical component is a waveguide.
 13. The process of claim 11wherein curing the optically transparent material and the underfillmaterial comprises simultaneously curing the optically transparentmaterial and the underfill material.
 14. The process of claim 11 whereincuring the optically transparent material and the underfill materialcomprises first curing the optically transparent material and thencuring the underfill material.
 15. The process of claim 11 whereincuring the optically transparent material and the underfill materialcomprises first curing the underfill material and then curing theoptically transparent material.
 16. The process of claim 11 wherein theoptically transparent material can be fully cured at temperatures lessthan or equal to 180° C.
 17. The process of claim 11 wherein theoptically active area is a detector or a source.
 18. The process ofclaim 11 wherein the optically transparent material has a low modulus ofelasticity.
 19. The process of claim 11 wherein the opticallytransparent material has a refractive index substantially the same as arefractive index of the optical component.
 20. The process of claim 19wherein the optically transparent material has a refractive index ofapproximately 1.5.